Taig
Micro Mill DRO Tachometer & SFM
Last updated on
Tuesday, September 19, 2023 04:19:19 PM
Mountain US Time Zone
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EO Sensor,
Encoded Pulley,
Infrared Reflectivity,
Surface feet per Minute
This DRO
modification has been
updated for the
CNC conversion
but this information has been retained for reference.
See updated
CNC Tachometer.
A
Fairchild QRB1114 E-O sensor
is mounted using a milled
Delrin plastic enclosure so the circuit will not short.
Mounted to the mill head using the two, pre-existing
10-32 tapped holes.
Alignment is easily accomplished
by moving the spindle up or down on the
dovetail mount.
The shielded power/signal cable exits to the left
through a heavily chamfered hole in the aluminum motor support.
I know what the fixed speeds are, though
it's easier to look at the
DRO RPM than
the chart,
but it's the SFM calculator that I find useful.
For the
DPU-550, a
74LS14 Schmitt Trigger would be
needed to make the
tachometer work reliably.
Diagram.
The IC was spliced into the AUX IN
to header wires & then shrink-wrapped
Pin 7 is ground, pin 14 is +5VDC, pin 1 is the signal IN
from the sensor &
pin 2 is the signal OUT to the DRO.
Encoded
Pulley
Did not need the black tape. The circuit easily
picks up a
pulse from the black set screw,
representing about a 3.5% duty cycle.
The sensor is 0.286" from the pulley
surface.
Sensor circuit enclosure with the cover removed.
The
two, flat-head screws are 3-48.
A nylon tie serves as cable strain relief &
the hot glue keeps it from rotating.
Infrared Reflectivity
A near-infrared (NIR) camera picture, using
the
Sony DSC-F717 night shot
mode, showing
the Fairchild NIR emitter glowing at 940
nanometers (nm) which is invisible to the
naked eye.
Objects that appear light or dark to the eye can exhibit
either
high or low
NIR reflectivity. This characteristic
must be taken into account when selecting materials for
use in NIR reflective
tachometer pick-up assemblies.
For example, if the pulley's black set screw had high
NIR reflectivity, it
would not have activated the
sensor properly. This principle applies to all
materials including: plastic,
metal, paint, tape, etc.
Selecting a material based solely on its visual
appearance can lead to a sensing
failure.
Surface
Feet per Minute
SFM is only a starting point. When cutting metal,
one attends to (among other things):
speed, feed,
chip size,
chip
length,
chip
coloration,
coolant,
rigidity,
surface finish, sound,
smell, &
vibration.
There are numerous, interacting variables that are
unique to
any given machine & setup that simply
can not be accounted for by SFM tables.
The rigidity, coolant, & feed in a vertical machining center
is a bit better than a hand drill. So to say that one
SFM value should be the
same for both is a stretch.
SFM = (RPM x pi x DIAMETER) / 12 where: pi = 3.14159 & the diameter is in inches
Surface Feet/Minute (SFM) Chart 1 SFM 2 SFM 3 Machinist Calculator
EO Sensor, Encoded Pulley, Infrared Reflectivity, Surface feet per Minute
EO Sensor,
Encoded Pulley,
Infrared Reflectivity,
Surface feet per Minute